The significance of adult buck aggression as a proximate factor driving yearling male dispersal in roe deer, Capreolus Capreolus, was studied in Sweden during the dispersal season (April–June) in 1988–1991. In roe there is a well established positive correlation between yearling antler size and dispersal tendency. I tested the prediction that yearlings with large antlers will experience more agonism from adult bucks than those with smaller antlers. Additionally, I studied the relationship between antler size and level of sexual maturity, by recording date of velvet shedding, and sampling testes weights from culled yearlings during the breeding season. The amount of agonism experienced by yearlings was positively correlated with the size of their antlers during April and May, whereas in June no such relationship was found. Aggression towards all yearlings tended to increase as the season progressed. A positive correlation between antler size and testes weight was evident, and the larger the antlers of yearlings the earlier the velvet shedding occured. These results indicate: firstly, that adult buck agonism is an important proximate factor underlying yearling male dispersal; and secondly, that agonism is mainly directed towards the sexually most mature yearlings. This, in turn, suggests mate competition to be the ultimate cause of male-male agonism, and thus male dispersal in this species.